7.1.3: multi-domain and smtp/TLS; manually change master.conf.in?

I have 7.1.3 OSS installed in a multi-server, multi-domain environment, with separate IPs and commercial certificates for each domain. IMAP/Web access with StartTLS works fine, but SMTP via submission port (587) with startTLS seems to be returning the wrong certificate.

testing with openssl s_client -connect <host : port> -starttls smtp -status, I can see that the only the base domain certificate is returned each time.